Concerns about mercury in dental amalgam have created a need to extend the use of composite resins to posterior applications. However, current composites are not suitable for large stress-bearing applications involving cusps due to excessive wear and fracture. The glass fillers in resins provide only limited reinforcement because of their brittleness and rounded shapes. Preliminary studies with ceramic whiskers having high strength and elongated shapes showed a two-fold increase in composite strength and toughness, and promising results on polishability, water absorption and strength durability, enamel wear and esthetics. The goals of this project are to i) to investigate the whisker reinforcement mechanisms for dental resins; ii) to understand the key microstructural and processing variables that determine reinforcing efficacy; and iii) to develop optimized prototypes. To accomplish these goals, the following studies are proposed: (1) the effects of whisker size and aspect ratio will be evaluated to improve reinforcing efficacy and filler packing; (2) dental silicate fillers will be fused onto the individual whiskers to minimize whisker entanglement, enhance silanization, and improve retention in matrix by providing rougher whisker surfaces; (3) the effect of whisker content will be systematically investigated on composite mechanical properties and reinforcing mechanisms, translucency, color and depth of cure; (4) the mechanical and physical properties of water-aged composites will be characterized to identify the reinforcing mechanisms and the key microstructural parameters, and this knowledge will be used as feedback to improve microstructural design; (5) three- and two-body enamel against composite wear will be evaluated to understand the effect of whiskers on wear mechanisms and wear rates; and (6) the whisker composite will be veneered with conventional composites and the effect of whisker reinforcement on properties of the final restoration will be evaluated by using Hertzian cyclic fatigue and flexural tests. This project will provide (1) an understanding of the effects of whisker- glass filler hybridization and fusion on composite properties; (2) mechanisms of whisker reinforcement for dental resins; (3) guidelines on microstructural design and processing of whisker-reinforced composite resins; and (4) a basis for the next generation of high performance composite resins for dental restorative and potentially other biomedical applications.